You can consider using a relay that will, when energized, connect the motor to the supply and when de-energized, connect the motor/generator to a 22 ohm 200 watt power resistor.
Your motor, when running idle will generate close to 180 volts. The 22 ohm resistor will consume around 8 amperes (the motor's rated current) and quickly stop it.
BTW, it will rotate for 6 minutes ???? Does it have a flywheel attached to it ? If so, consider a much larger 22 ohm brake resistor. That resistor will have to absorb and dissipate all of the flywheel's energy and a 200 watt resistor may become red hot !